You should memorize all 15 of the brightest stars visible from your location.

Looking at Stars

All stars are so far away that they appear thousands of times smaller than a pinpoint.

They look a lot bigger than they are because the light triggers one cell in your eye, which is the smallest thing
you can see.

Stars twinkle because as the narrow beam of light traverse the air, it can bend and miss your eye.

If you are using a telescope, try picking a night when the stars do not twinkle much. Called good "seeing."

You can see dim stars by looking several degrees away from them, using peripheral vision, because
the place in your eye that focuses best is not the most sensitive to light.

Several stars are bright enough to see their actual colors, especially as red, orange, yellow, white and blue.

Photographs of Stars

Bright stars make big images in photos, but that image is not a picture of the star.

A big star image is formed because the light gets smeared out in a circle.

The rays sometimes seen coming from stars in photos are images of the secondary mirror supports.

The rings and disks around stars images are caused by diffraction.

Astronomers do not use photographs much to measure star properties.

Some colors in photographs are not accurate, but a caused by film failures. The Orion Nebula
looks red in photos (from Hydrogen's H-alpha line) but looks green to the eye (from Oxygen).

Distances

The light year (ly) is the distance light travels in one year, being about six million million miles.

The nearest star is over four ly away. The light from the sun takes about 8 minutes to reach earth.

Astronomers also use the par sec (pc), which is the distance of a star with a parallax of one arc second.

No star is as close as one par sec, but is close to that, being 1.3 pc. Stars are about 1 pc apart in our stellar neighborhood.

1 pc = 3.26 ly.

Apparent Brightness of Stars

Apparent brightness is how bright a star appears to be, whether or not it is a nearby star, or extremely distant.

We still use the Greek system of apparent magnitudes.

The brightest stars were said to be first magnitude.

The dimmest stars visible in a dark sky far from a city are sixth magnitude.

The dimmest stars visible in a city are often only third magnitude.

There are fifteen first magnitude stars visible from the U.S. You should know their names and locations.

All of the stars in the Big Dipper are second magnitude stars.

The four stars in the bowl of the Little Dipper are of 2nd, 3rd, 4th, and 5th magnitude, so they are a good
measuring stick for comparisons.

Modern astronomers define magnitudes so that a difference of five magnitudes is a factor of 100 in
brightness. That means that some very bright stars have 0 magnitude, or even negative. Sirius has magnitude -1.

Venus has a magnitude of -4, the moon is about -12, and the sun is -26.

Measuring Properties of Stars

Absolute Magnitude

Absolute magnitude is defined to be what the apparent magnitude would be at a distance of 10 pc.

It measures the "absolute" brightness of a star, that is, how much light it is really emitting.

The absolute magnitude of the sun as about +5.

The absolute magnitude of Rigel is about -7, meaning it is really incredibly bright.

Many of the stars visible in the sky are the bright beacons. There are many closer stars too dim to see.

Absolute magnitude usually means the brightness at all wavelengths, also called absolute bolometric magnitude.

If one wants to refer only to wavelengths of visible light, then it is the absolute visual magnitude.

Spectra

A sprectrograph spreads the stars light into all of its component colors onto photographic film.